Efficient digital data utilization of limited frequency band channels ordinarily used for telephone voice transmission is accomplished by multilevel high speed signals, making possible, for example, transmission at 9600 bits per second. Low error rates, however, can only be achieved by effective compensation of signal degradation otherwise highly destructive to data signals. This degradation, which is produced by typical transmission channels, is widely tolerated since it is innocuous to voice transmission. Linear distortion due to variations in the attenuation and the delay imparted to different frequency components produces a dispersion effect, which degrades the transmission signal, commonly known as intersymbol interference. Transversal filters are generally used to implement adaptive equalizers, which are an effective means of compensating for intersymbol interference.
In addition to linear distortion, there are also carrier-phase perturbation effects which are harmful to digital data signals. One source for such effects is the frequency-translation oscillator of a frequency division multiplex (FDM) channel bank. The low-level, but omnipresent, power supply ripple in these oscillators contributes phase-jitter to the digital data signal at the fundamental and other low-order harmonic frequencies of the AC power frequency. Other sources of carrier phase perturbation are random fluctuations of carrier phase, incidental FM, and/or phase jitter. As a result, the spectral components of the phase-jitter encountered in transmission systems may range anywhere from the harmonics of the AC power frequency down to almost zero frequency.
In U.S. Pat. No. 3,878,468 issued to D. D. Falconer et al on Apr. 15, 1975, an arrangement utilizing a phase-jitter compensator connected to the output of an equalizer is disclosed for providing jitter-free passband equalization of data signals. This arrangement is an example of what is commonly referred to as a first order data-directed phase-locked loop. The logical extension to achieve a performance advantage, such as for frequency offsets, would be to utilize a second order phase-locked loop. For both orders of these conventional data-directed loops, the parameters are intrinsically fixed and selected to achieve a compromise between a bandwidth wide enough to track the highest frequency jitter and yet not too wide so as to reduce the noise passed to the loop output. Because the presence and degree of phase jitter varies from connection to connection, first and second order phase-locked loops are conservatively designed to track only the lower frequency components of the phase jitter leaving the higher frequency components present to degrade the demodulated signal.
A primary object of this invention is to provide a frequency-selective phase tracker capable of adaptively tuning to the frequency components of carrier phase perturbations while simultaneously minimizing the noise in the estimate.
A related object is to provide a frequency selective phase compensator that has a nonrecursive structure for adaptively minimizing mean-squared angular error in a unimodal fashion.